While Ellis, Reid, and Kramer’s life history framework is convincing in explaining individual variation in life history strategies, it has certain limitations, which we discuss – drawing notably on the economic theory of fertility choice. In particular, we suggest that ambient cues to extrinsic mortality alone are not sufficient to trigger sustained net fertility decline.

The two-tiered model offers a meaningful advance in life history theory, but refinements are needed to enhance testability. We highlight challenges related to the timing of Tier 1 and 2 effects and potential curvilinearity in Tier 2 responses. Clarifying these dimensions would improve falsifiability and strengthen the model’s utility for guiding empirical research.

This commentary expands Ellis et al.’s 2-tiered life history (LH) model by integrating shyness and insecure attachment as mediators of environmental adaptation. Shyness balances survival-reproduction trade-offs with mixed LH outcomes. Avoidant attachment accelerates LH strategies under harsh conditions; anxious attachment delays reproduction under unpredictable conditions. Incorporating social behaviors, which are related to survival and safety, enhances the model’s applicability across behavioral domains.

Harsh extreme-Earth and extra-Earth mortality sources pressure countervailing shifts in human life-history traits and survival-reproductive strategies, trading shorter lifespans and reproduction spans to hedge scarce gametes with sex-chromosome resilience. Darwinian trajectories for genotypes and phenotypes nonetheless remain unknown for future Earth-population sustainability and deep-space colonization. Controversial technology-assisted human evolution may be needed to narrow anthropological evolutionary-medicine disparities and prevent Humanity’s extinction.

Hierarchical response models are advancing our understanding of the facultative adjustment of development and reproduction to the environment. E, R, and K’s manuscript brings together a vast array of empirical data to support their Tier 2 model highlighting the importance of both the energetic and local mortality environments for reproductive development and timing. We propose that incorporating existing hierarchical response models, as well as analyses from industrial societies, will aid the hunt for underlying mechanisms that embody the local mortality environment impacting reproductive schedules.

Environmental factors do not directly drive life history outcomes. Rather, their influence is mediated by cognitive and affective processes. Here we review several psychological processes, which mediate the influence of environmental mortality on life history strategies in both Tier 1 and 2 from the target article.

The number and timing of births are strongly associated with the stability of available resources and the risk of extrinsic mortality. The authors suggest a verbal model to disentangle the relationship between these two variables. However, we challenge their assumption of a hierarchical relationship between these two predictors and propose that formal modelling of their interaction, within a developmental perspective, could further our understanding of how the reproductive strategies of women respond to environmental uncertainty.

ER&K recurrently emphasize one side of three equations: (1) extrinsic rather than intrinsic mortality pressures; (2) developmental rather than genetic influence; and (3) individual rather than population-level effects. Furthermore, they insufficiently disambiguate extrinsic from intrinsic mortality, especially confounding how mortality evolutionarily constrains and developmentally calibrates genotypic life history speed, and struggle to explain population-level differences in fast maturation among modern populations.

The two-tiered model of developmental plasticity is elegant and presented with impressive interdisciplinary synthesis. We suggest that yet more – social context and nutrition behavior – will need to be incorporated into empirical research. Drawing from anthropology, nutrition, and neuroscience, we highlight connections that may help generate new approaches for studying the developmental calibration of life history in humans.

The force of selection on survival changes with age and models of reproductive effort (RE) reflect this finding that increases in infant and adult mortality have different effects on optimal effort. Using an age-structured demographic model, I show that infant mortality constrains selection on the human life cycle. Effort directed at increasing infant survival is strongly favored over efforts to increase adult survival. Complete accounts of human reproductive decisions require age-specificity of effects.

This commentary complements Ellis et al.’s two-tiered framework by using panel data and emphasizing the role of population density in decelerating life history (LH) strategy. We show that mortality cues and energetic constraints do not operate in isolation, demonstrating how density-dependent competition shapes LH strategies over time and underscoring the need for a dynamic, multifaceted approach to LH development.

Ellis and colleagues propose that plasticity in human life history strategy (pubertal timing, reproductive pacing) is governed by energetic and mortality-cue related regulatory tiers. We feel that our published interpretation of these findings – that growth (and thus maturational timing) is energetically driven, while reproductive behaviors are largely free to vary independent of energetics – provides a more parsimonious interpretation of this work. The concept of tiers is mechanistically vague and introduces unnecessary complexity to this literature.

Simulations, correlational studies, and laboratory experiments have recently shown that cues to high-resource competition may be steering individuals toward increasingly less favorable attitudes toward reproduction and slower life-history strategies. This work suggests that, alongside extrinsic mortality, it may be fruitful to examine the role of competitive stress cues in shaping life-history strategies.

The featured article introduces a much-needed theoretical framework for developing a dual-process model of life history calibration. This model accounts for the counterbalancing effects of individual energetic stresses and extrinsic mortality threats of the environment. This framework also reinstates resource availability – a key determinant of energetic conditions – into life history research, resolving its previous exclusion due to similar countervailing influences relative to extrinsic mortality threats.

Our systematic analysis of operationalizations and conceptualizations of harshness (extrinsic mortality) and unpredictability in the (psychology) life history literature highlights that (1) employment of extremely diverse measures contributes to the confusion about the effect of harshness on life history traits and (2) few measures reflect energetic stress or ambient EM, such that Ellis et al.’s two-tiered model should motivate future research.

The field of life history “theory” is in a state of heightened entropy; the paper by Ellis, Reid, and Kramer provides a proposal for structuring future research on this topic. I find their proposal a constructive one and describe its potential benefits. Furthermore, I propose several ways in which we can additionally develop the life history framework.

Building upon the two-tiered model of life history strategies introduced by Ellis, Reid, and Kramer (2024), we discuss avenues for future research that apply the theory to sexual and gender minority individuals. We propose that minority stress and suicide exposure may serve as extrinsic ambient mortality cues among sexual and gender minority (SGM) populations. We also identify a feature of pubertal development requiring more consideration.

After presenting their two-tier model, Ellis, Reid, and Kramer (2024) tentatively state that child mortality risk should predict offspring quantity, while adult mortality risk should accelerate reproductive timing. I test these assertions using mortality risk and fertility data from the Global Burden of Disease (GBD) project for 204 countries and territories over 32 years, finding partial support for their predictions.

Ellis et al.’s two-tiered model compellingly integrates biological and psychosocial pathways through which extrinsic mortality shapes life history strategies. An understated, yet important, implication of their framework is the role of social inequalities in signaling mortality risk – particularly through differential exposure to bereavement – and how it may give rise to distinct life history strategies and outcomes.

In response to recent arguments for a one-tiered model of life-history development, Ellis and colleagues present a two-tiered model that integrates energetics with development. Early formulations of this model were, however, already two-tiered. The target article separates itself from earlier work by digging deeper into the underlying energetics assumption and how it transacts with ambient cues to impact development.